Zinc oxide crystal bulk-effect oscillator and method of preparing the same

ABSTRACT

TO PROVIDE A COMMERCIAL ZINC OXIDE SINGLE CRYSTAL WITH PREDETERMINED ELECTRICAL CHARACTERISTICS IT IS CLEANED, EMBEDDED IN ZINC OXIDE POWDER, AND HEATED (E.G., FOR 1-3 HOURS AT 750*-800*C.) IN AN ATMOSPHERE HAVING A PREDETERMINED OXYGEN PARTIAL PRESSURE (E.G., ONE PART OXYGEN TO 10,000 PARTS ARGON). TEMPERATURE AND OXYGEN PARTIAL PRESSURE ARE CONTROLLED TO DETERMINE THE RESISTIVITY CHARACTERISTICS. TO SHORTEN SUCH TREATMENT THE CRYSTAL CAN BE SUBJECTED TO A PRELIMINARY HEAT TREATMENT IN AIR OR OXYGEN AT NEAR-ATMOSPHERIC PRESSURE AT ABOUT 750*C. FOR 3-18 HOURS.

United States Patent 3,671,193 1 ZINC OXIDE CRYSTAL BULK-EFFECT OSCILLA- TOR AND METHOD OF PREPARING THE SAME Stanley V. Jaskolski, N76 D22144 Chestnut Hill Road, Sussex, Wis. 53089, and Martin A. Seitz, 9133 W. Custer Ave., Milwaukee, Wis. 23225 No Drawing. Filed Feb. 11, 1970, Ser. No. 10,635 Int. Cl. C01g 9/02; G03c 5/00 US. Cl. 23-148 5 Claims ABSTRACT OF THE DISCLOSURE To provide a commercial zinc oxide single crystal with predetermined electrical characteristics it is cleaned, embedded in zinc oxide powder, and heated (e.g., for 1-3 hours at 750-800 C.) in an atmosphere having a predetermined oxygen partial pressure (e.g., one part oxygen to 10,000 parts argon). Temperature and oxygen partial pressure are controlled to determine the resistivity characteristics. To shorten such treatment the crystal can be subjected to a preliminary heat treatment in air or oxygen at near-atmospheric pressure at about 750 C. for 3-18 hours.

This invention relates to a method of treating commercial undoped zinc oxide single crystals to insure that they will have certain electrical characteristics which such crystals in their untreated state possess only very rarely and inconsistently, if at all; and the invention relates more particularly to a method of treating such crystals, without doping them, to cause them to have desired electrical characteristics and to control values of the characteristics they are caused to have.

It has been known heretofore that an undoped Zinc oxide single crystal possessed certain unusual electro-resistive properties at very low temperatures, although there was no consistency in the values of those properties from crystal to crystal, and it was only very rarely that those properties were found at higher temperatures.

Specifically, it was known that the electrical current through these rare zinc oxide crystals varied substantially linearly with applied voltage up to a certain critical value of voltage, but that above that critical value, current flow through the crystal displayed a non-ohmic behavior, being below the normally expectable ohmic value for a given increase in applied voltage, and at still higher applied voltages passing into a regime in which current oscillations occurred with a constant applied voltage. These results were heretofore obtained only at temperatures on the order of 77 K. (Meyer & lorgensen, 20 Physics Letters, No. 5, p. 450; Wettling & Brunn, 27A Physics Letters, p. 123.) Furthermore, the electric field thresholds (voltage divided by length) of the non-ohmic resistivity regimes varied from crystal to crystal in the crystals that manifested the effect.

Electroluminescence was also noted under some condition, but this effect was observed only with some specimens and therefore information concerning it was to some extent controversial (Meyer, lorgensen & Balslev, 3 Solid State Communication, p. 393).

It was evident that the peculiar electrical properties of these rare zinc oxide crystals would have made them useful for many practical purposes. But it was also evident that there was no point in contemplating any practical application of such crystals, or even intensive research upon them, so long as the quantitative performance characteristics of such crystals varied so widely from specimen to specimen as to preclude any possibility of obtaining predictable and repeatable results. The fact that the peculiar resistivity characteristics mentioned above were 3,671,193 Patented June 20, 1972 consistently obtained only at the temperature of liquid nitrogen was of course a further very important deterrent to practical application.

The present invention has for its general object to provide a method of so treating zinc oxide single crystals as to insure that they will uniformly possess at room temperatures the peculiar electrical properties heretofore found only at extremely low temperatures, and which method makes crystals having these properties available in large numbers and at low cost so that their potentialities for practical applications can be readily realized.

It is also an object of this invention to obtain the properties just mentioned in a commercially pure zinc oxide single crystal without doping it, that is, without deliberate addition of impurities to it.

A zinc oxide crystal treated by the method of this invention exhibits ohmic behavior up to a certain critical value of applied electric field (volts per centimeter); that is, current flow through the crystal increases proportionally to applied field until the critical value is reached. When the applied electric field exceeds the critical value, the crystal exhibits a non-ohmic behavior, manifested by a current-saturated current-field characteristic, and the current through the crystal develops instabilities in the form of oscillations. The oscillations occur in the frequency range of 1 to 20 mHz., at adjustable power levels which are known to be as high as 5 watts delivered into a 200 ohm load and which, it is believed, can substantially exceed that level.

The threshold or critical value of electric field at which current oscillations occur in a given crystal is temperature dependent, although the variation of threshold value with temperature is not a large one through the range of inhabitable atmospheric temperatures. Sub-threshold resistivity is similarly temperature dependent.

At room temperature the threshold electric field value for any zinc oxide crystal treated by the method of this invention is about 2,000 volts per centimeter, irrespective of the sub-threshold resistivity of the crystal, providing its sub-threshold resistivity is within the range considered to be most useful. The useful range of sub-threshold resistivity is regarded as between about 1 ohm-centimeter and about ohm-centimeters. Inasmuch as the threshold electric field value is substantially independent of resistivity within the range just mentioned, the current at saturation will depend upon the sub-threshold resistivity of the particular crystal. The reason for favoring the above mentioned range of sub-threshold resistivities is that with values of resistivity above that range not enough power can be taken out of the crystal for most foreseeable practical applications of the crystal as an oscillator, while at values below that range it appears that the amount of power that must be put into the crystal to develop useful current oscillations is so great that the crystal is in danger of being burned out.

With the foregoing in mind, it is another and very important object of this invention to provide a method whereby an undoped commercial single crystal of zinc oxide can be treated to provide it with a desired subthreshold resistivity at room temperature (or any other desired temperature) and a corresponding critical electric field threshold value, or, alternatively, to provide it with a desired saturation current density value and corresponding sub-threshold resistivity. Since resistivity and conductivity are reciprocals of one another, it could also be said to be an object of this invention to provide a treatment for zinc oxide single crystals which causes them to have substantially predetermined sub-threshold conductivity values.

The present invention rests upon the recognition that the electrical characteristics of an undoped zinc oxide single crystal, and particularly its resistivity, are mainly 3 P e dependent upon the number and distribution of defects in the lattice of the crystal, and the invention resides in the provision of a method of producing in a commerc1al single crystal of pure zinc oxide a predetermined concentration of such defects, uniformly distributed through the crystal.

Impurities in a zinc oxide single crystal can be considered defects, and they have some influence upon the electrical characteristics that a crystal will manifest. However, commercially available zinc oxide single crystals that contain less than fifty parts per million of impurities are the ones with which this invention is concerned, and in crystals of that purity the influence of the impur1t1es upon the electrical characteristics of the crystal has been found to be relatively very minor, especially when the impurities are distributed substantially uniformly through the mass of the crystal.

Thus the defects which constitute the main concern of this invention are defects in the nature of zinc atoms located interstitially in the lattice structure of the crystal, and it can also be said to be the object of this invention to provide a method by which a commercially available zinc oxide single crystal, having less than fifty parts per million of impurities, can be so treated without doping it as to cause the crystal to have such a concentration of interstitially located zinc atoms in its lattice structure, uniformly distributed through the structure, as will endow the crystal with a selected one of a family of values of electrical properties.

It will be recognized that certain of the electrical properties made available by the method of this invention are related to the piezoelectric character of the zinc oxide crystal. In theory a zinc oxide crystal should exhibit piezoelectric characteristics because of its asymmetrical lattice structure, but heretofore the piezoelectricity of undoped zinc oxide crystals has been effectively masked by their extremely low conductivity. Hence it is another object of this invention-achieved by reliably endowing undoped zinc oxide single crystals with the above described physical characteristics (defect concentration, resistivity, etc.)to enhance the utility of the observable piezoelectric properties of such crystals to the point where it becomes a characteristic of them that can be used for certain practical applications.

It will be seen that it is an ultimate object of this invention to make available undoped zinc oxide single crystals that reliably possess at room temperatures as well as lower temperatures the electrical properties described above; and, moreover, to provide zinc oxide single crystals that are quantitatively standardized with respect to such properties, to thus make feasible the exploitation of those properties in practical applications which can be readily envisioned by those skilled in the art, including avariety of oscillator, transducer and amplifier devices, and others which are only vaguely foreshadowed but which will undoubtedly be developed with increasing availability of solid state devices possessing the peculiar properties in question.

While crystals of cadmium sulfide are known to possess certain of the electrical properties obtained by treating zinc oxide crystals according to the method of this invention, the high frequency AC. power level of a treated zinc oxide crystal is on the order of 1,000 times that of a cadmium sulfide crystal for a given D.C. input; hence it is another object of this invention to provide a crystal havpiezoelectric properties suitable for a bulk-effect acousto electric oscillator, which crystal can generate levels of AC. power much higher than the best crystals heretofore available for the same purpose.

With these observations and objectives in mind, the manner in which the invention achieves its purpose will be appreciated from the following description, which exemplifies the invention, it being understood that such changes in the precise method of practicing the invention may be made as come within the scope of the appended claims.

Commercially supplied undoped zinc oxide single crystals are used for the process of this invention, as distinguished from polycrystalline zinc oxide. Such crystals are grown by the vapor transport process or a hydrothermal process, and are available at 99.999% purity, that is, they contain less than 50 parts per million of elements other than zinc and oxygen.

in general, the method of this invention comprises certain preparatory steps by which the crystal is cleaned to free it from external contaminants and is otherwise made ready for heat treating; a heat treatment step by which interstitial zinc atoms are obtained in the crystal in a desired concentration and uniform distribution and by which a uniform dispersion of impurities through the crystal is also obtained; post-heat cleaning operations; and, finally, installation of contacts. In a modified version, the method includes a preliminary heat treatment step in which the, crystal is homogenized and is brought to a substantially stoichiometric strain-free condition, to permit the main heat treatment step to be shortened.

The preparatory steps are directed to bringing the crystal to be processed as nearly as possible to perfect cleanliness and freedom from external contaminants, and to correcting its geometry and orientation.

First the crystal is lapped or ground on all surfaces, to bring its surfaces to flatness and to parallelism with one another and to the desired orientation with respect to the longitudinal (C) axis. For this purpose it is preferred to use a fine grit grinding agent, such as No. 600 grit silicon carbide abrasive particles (25 micron size) in a fluid vehicle such as oil or water.

As a part of the lapping or grinding step, the ends of the crystal are brought to flatness and parallelism at a desired angle to the C-axis of the crystal. Theory indicates, and experiments appear to confirm, that best results are obtained with the end surfaces at an angle of 35 to the C- axis.

After grinding, the crystal is washed in cold acetone or a similar grease solvent that evaporates readily without leaving a residue, to remove all grit and grease from the crystal.

Next the crystal is washed in a mild etching acid which will remove surface layers of the crystal molecules, including material near the surface that has been strained by previous handling of the crystal. External contaminants are of course carried away with the material thus removed.

A moderately dilute phosphoric acid (about 20% solution) is preferred for this etching operation because it is easily obtainable, is relatively mild, and reacts with zinc oxide to form zinc phosphate, which does not adhere to the crystal. This etching step can be conveniently accomplished by immersing the crystal in the acid for a few minutes. The acid can be moderately heated to expedite the operation, but it will be understood that etching is not continued for a long enough time to permit any substantial penetration into the crystal of the elements constituting the acid.

In final preparation for heat treatment, the crystal is placed in a boat or tray of a material such as aluminum oxide, in which it is completely surrounded by powdered zinc oxide of high purity (99.999%). The boat or tray.-

provides a container in which the crystal and its zinc oxide powder embedment can be handled and transported. The material of the boat is not critical, so long as it is one that has stable stoichiometry at temperatures up to about 1000 C., does not react with oxygen, zinc or the impurities common to commercial zinc oxide crystals, and does not give off any contaminants at temperatures below about 10 00 C. It will be evident that the boat could be of quartz or of certain ceramics, for example, rather than of alumina, but boats of alumina have been consistently satisfactory and hence for reasons of cost and conven ence there seems to be little reason to use a boat of any other material.

The zinc oxide powder in which the crystal is embedded provides a buffer layer through which impurities from the alumina boat must diffuse before they can reach the crystal, but which does not interfere with transfer of oxygen to and from the crystal.

During heat treatment, such impurities as are present in the crystal are brought to a uniform dispersion through it, by reason of maintenance of the crystal at an elevated temperature for a prolonged period; and for the same reason the crystal is relieved of mechanical strains that may have been produced during its growth, as the result of imperfections in its lattice structure, or may have developed from subsequent handling. Furthermore, by reason of the atmosphere in which the crystal is maintained and the temperature of the system comprising the crystal and that atmosphere, there is produced in the crystal during the heat treatment step a controlled concentration of interstitial zinc atoms, uniformly distributed through the crystal, whereby the crystal is caused to have a desired resistivity.

With the crystal embedded in zinc oxide powder, it is placed in a single zone furnace in which there is maintained an atmosphere that is a mixture of chemically inert gas and pure oxygen, the proportions of these gases being selected, as described hereinafter, in accordance with the resistivity desired for the finished treated crystal. The temperature maintained during the heat treating step also depends upon the resistivity desired. The duration of the heat treatment can be varied within certain limits, for smaller crystals, as an alternative procedure for controlling resistivity.

Before taking up examples of practice of the process on an empirical basis, its theoretical foundation will be explained.

To obtain the desired resistivity in the finished treated crystal, the temperature and oxygen partial pressure are controlled, during the heating step, in accordance with principles of chemical thermodynamics.

The processes that occur during the heating step are reversible ones:

where Zn, denotes a neutral zinc interstitial atom in the zinc oxide lattice; Zn, denotes an ionized zinc interstitial atom; gas is denoted by (g); and e denotes an electron.

At any particular temperature the equilibrium constant K for the first of these reactions is given by:

Since the oxygen partial pressure P of the furnace atmosphere is controlled during the heat treating step to control the value of the equilibrium constant K the value of [O (g)] in Equation '2 can be taken as the oxygen partial pressure P and where K designates the equilibrium constant for the thermodynamic standard reference state, In designates natural logarithm (log E is energy of formation of zinc interstial atoms, k is Boltzmanns constant and T is temperature in absolute units (degrees Kelvin).

With respect to the second of the two above expressed reversible reactions, most of the free electrons in the finished treated crystal come from zinc ion interstitials,

and therefore It follows that the equilibrium constant for this second reversible reaction is given by The variation of the equilibrium constant for this reaction with temperature is given by K =K ln( where K designates the equilibrium constant for the thermodynamic standard reference state, and E is ionization energy of the zinc interstitial atom, numerically equal to .05 electron volts.

From Equation 3 Combining this with Equation 6 Rearranging: n= /K K (P (10) From the foregoing equations it is possible to calculate the temperature and the oxygen partial pressure of the atmosphere that are required to be maintained during heat treatment in order to achieve a desired defect concentration, and hence conductivity, in the processed crystal. This can be done on the basis of the relationship G=1/p=ne (11) where C denotes conductivity, p denotes resistivity, n is defined by Equation 10, e is the charge on an electron (numerically, 1.6 10- coulombs) and a is the mobility of electrons, which at room temperature is approximately 200 cmfi/volt-sec.

For the results to be in accordance with the calculations, heat treatment must theoretically be continued for a long enough time to assure that the reaction attains equilibrium and the crystal attains homogeneity. The duration of heat treatment could be lengthened indefinitely, but at the obvious expense of production efficiency and at the possible risk of driving surface contaminants into the crystal.

In practice, argon has been used in the atmosphere for heat treatment because it is chemically inert and is the cheapest of the chemically inert gases. Furthermore it is easy to control the oxygen partial pressure in an argonoxygen atmosphere. As an example, a mixture of one part of oxygen to 10,000 parts of argon at atmospheric pressure has an oxygen partial pressure of 10- atmos. In the temperature range 750850 C., this partial pressure of oxygen was found to give a desirable defect concentration and conductivity.

At the conclusion ofheat treatment the crystalis withdrawn from its zinc oxide powder embedment and is thoroughlycleaned, preferably by again subjecting it to a mild acid bath and bathing it in acetone for removal of grease, grit and foreign material.

- The crystal is then readyto have contacts applied to itsends. Preferably the contacts are of indium, which makes a good low resistance ohmic contact (as distinct from a high resistance non-ohmic contact) and which has the advantage of melting at about 200 C. so that the contacts can be readily vapor-deposited or soldered on the ends of the'crystal.

In a modification of the method of this invention,- the crystal, after being lapped, etched and embedded in powdered'zinc oxide, is subjected to a preliminary oxidizingand homogenizing heat treatment before being sub-. jected to the heat treatment by which itselectrical properties are established at the desired values. V I

For thispreliminary heat treatment, the boatcontainingthe crystal embedded in zinc oxide powderisplaced' in a single-zone heat treating furnace in which isxmaine tained an air atmosphere at atmospheric pressure, or a pure oxygen atmosphere at a near atmospheric pressure, and in which a temperature on the order of 750fC is maintained for a period of time 'on the order of 14 to 18 hours.' a

:-In general, the first heatingstep should be continued long enough to insure strain relief and homogenization of impurities in the crystal, but it should notbe continued so long that impurities that may be present on or near the surface of the crystal diffuse into its interior.

At the end of the preliminary heat treatment the crystal will be homogeneous, that is, such impurities as it contains will have diffused uniformly through it, the lattice structure of the crystal will be substantially free fromstrain, and the material will have been driven toward a near-stoichiometric condition.

If a zinc oxide crystal could be obtained that was absolutely pure, with no trace of any other element than zinc and oxygen, and which was wholly instoichiometric condition, it would be a nonconductor of fairly high dielectric strength. This is because a high level of energy (3.2 electron volts must be applied to a zinc atom within such a lattice structure to ionize it, whereas only .05 elec tron volt is neededto ionize an intersitialzinc atom.,

Crystals tested after being subjected to a proper performance of the preliminary heat treatment'stepfexhibit a relatively low conductivity (high resistivity) such as would be accounted for if the majority of the defects in the crystal were the inevitable traces of impurity in it. Moreover, this low conductivity is quantitatively quite uniform from crystal to crystal, and through different portions of individual crystals, confirmingthat the impurities have beenbrought to a state of substantailly uni: form dispersion through the crystal.

The advantage of performing the preliminaryheating step is that the crystal goes into the mainheating step relieved of strains and with a uniform dispersion of impurities and defects. The main heating step can thus be somewhat shortened and standardized, as to itsduration,

for all crystals that have been subjected to "the prelirnin ary heat treatment. 1, n

From experience it has been learned that itherelation' of temperature, time and oxygen partial pressure. that is suggested by theoretical calculations for obtaininga given value of sub-threshold resistivity can be somewhat modified, particularly with crystals having a cross sectional area not larger than about .04 to .05-emi With .such small crystals the heating step canfbe interrupted before the system comprising the crystal and the coiitrollcdat: mosphere attains equilibrium, and the'distribution of interstitial zinc atoms and other impurities through. the crystal body, while perhaps falling short of perfect uniformity, is nevertheless so uniform for practical purposes that departures from homogeneity cannot, be detected 8 Stopping the reaction short of equilibrium on the basis of empirically derived data is of course advantageous becauseitconserves processing time.

Thefollowing table presents data for twelve crystals of .05 cm. or less cross-sectional area, processed with a single heat treatment step in accordance with the principles of this invention, the heat treatment operation being terminated, in each instance before the reaction system had. attained equilibrium. In each instance the crystal was prepared as abovevdescribed, including the envelopment of the crystal in commercially pure zinc oxide powder. The tabulation therefore represents twelve specific examples of the practice of the method of this invention. In each instance-the atmosphere in which the crystal was heat treated was a commercially obtained mixture of oxygen and argon. The ratio of oxygen to argon was controlled to give the indicated oxygen partial pressure at a total pressure of one atmosphere. The resistivity values given are of course for the sub-threshold regime, at room temperature.

"From the consistency of these results, it is apparent that within obvious limits, interpolation is feasible for obtaining any desired value of resistivity and for achieving a' desired value of resistivity at a time-temperature combination that makes for optimum processing economy. It must be borne in mind, however, that these given values, and interpolated values based upon them, cannot be expected to be valid for crystals having a cross-sectional area larger than about .05 cm. However, the foregoing table of values will be useful for expediting an empirical approach to the processing of larger cross-section crystals.

From the foregoing description it will be apparent that this invention provides a method of so treating commercial grade undoped single crystals of pure zinc oxide as to cause them to possess predetermined electrical properties, so that the peculiar electrical properties heretofore posses's'ed only by extremely rare specimens of such crystals are reliably obtained, and the practical utilities of such properties can be realized. The invention also provides a novel solid state electro-acoustical oscillator capable of generating much higher levels of AC. power than those heretofore available.

T hose skilled in the art will appreciate that the invention can be embodied in forms other than as herein disclosed for purposes of illustration.

The invention is defined by the following claims:

A method of treating a commercial single crystal of substantially pure zinc oxide to insure that an oscillating cu'rrentwillbe produced in it upon its being subjected to an electric field in excess of a threshold value and that at electric fields below 'said value it will have a predetermined resistivity, which method is characterized by:

I (A) cleaning the crystal to insure that the crystal as a whole has less than 50 parts per million of impurit and.

(B) heating the crystal (1) in an atmosphere in which oxygen is present .at a partial pressure that is maintained constant at not in excess of about 10 atms., and which atmosphere is maintained free from reacting gases other than zinc and oxygen,

(2) at a temperature of the crystal and atmosphere which is maintained constant at more than about 500 C.,

(3) the oxygen partial pressure of said atmosphere and said temperature being in such relation to one another, in accordance with the law of mass action, as to maintain an equilibrium reaction with an interstitial zinc atom concentration in the crystal such that its sub-threshold resistivity is between about 1 ohm-cm. and about 100 ohm-cm, and

(4) the heating in said atmosphere being maintained for not less than about minutes, and long enough to insure that reaction equilibrium is established through substantially the entire mass of the crystal and that interstitial zinc atoms are substantially uniformly distributed through it.

2. The method of claim 1, further characterized by:

(A) before heating the crystal, embedding it in powdered substantially pure zinc oxide; and

(B) maintaining it so embedded while heating it.

3. A crystal of zinc oxide that has been treated by the process defined by claim 1.

4. A method of treating a commercial single crystal of substantially pure zinc oxide to insure its having useful electrical properties which are characteristic of piezoelectric materials, which process is characterized by:

(A) cleaning the crystal to insure that crystal as a whole has less than 50 parts per million of impurities;

(B) establishing an equilibrium reaction system comprising the crystal and oxygen,

(1) which system is substantially free from reacting chemical species other than zinc and oxygen, and

(2) in which system the oxygen is at a constant pressure which is on the order of 10- to 10- atmos, and

mp1s; 252M629 (3) which reaction system is maintained at a constant temperature which is in the range of about 500 C. to about 1650 C.; and

(C) maintaining said equilibrium reaction system for a period of time of at least about 15 minutes, and long enough to insure that reaction equilibrium is established through substantially the entire mass of the crystal and that interstitial zinc atoms are substantially uniformly distributed through it.

5. The process of claim 4, further characterized by:

after cleaning the crystal but before establishing said reaction system,

(A) establishing a preliminary reaction system comprising the crystal and oxygen and which is free from other reacting elements, the pressure of the oxygen in said system being constant and on the order of the partial pressure of atmospheric oxygen;

(B) heating said preliminary reaction system to a constant temperature of not less than about 500 C. nor more than about 1050 C.; and

(C) maintaining said heating of the preliminary reaction system for a period of time of not less than about one hour.

References Cited UNITED STATES PATENTS 2,259,409 10/1941 Wenzel et al. 23--148 3,104,365 9/1963 Broser et al. 23148 OSCAR R. VERTIZ, Primary Examiner H. S. MILLER, Assistant Examiner US. Cl. X.R. 

